1. Field of The Invention
This invention relates generally to silicone resin compositions, and more specifically to a photocurable silicone gel composition and precursor thereof, and to a method of making same.
2. Description of The Related Art
In the field of silicone chemistry, a variety of silicone compounds, polymers, and formulations have been developed for applications including sealants, conformal coatings, potting materials, and the like. Among the numerous silicone systems developed to date, a wide variety of moisture-curing silicone compositions have evolved, which depend for their cure on atmospheric humidity and/or moisture present on the substrate to which the composition is applied. Although such moisture-curing silicone systems provide good physical properties and performance when fully cured, they suffer the disadvantage that the moisture curing process is relatively slow.
In consequence, effort has been focused in recent years on silicone compositions having other curing modalities which are significantly faster than the moisture-curing process. In particular, photocurable (e.g., UV-curable) silicones have been developed which offer a fast curing speed, far superior to conventional moisture-curing silicones.
In one such UV-curing silicone system which has come into wide usage, a photocurable silicone polymer is prepared by reacting (a) an isomeric mixture of acryloxypropenyldimethylchlorosilane, comprising the isomers: ##STR3## wherein the isomer (I) is present at a concentration of about 75%-80% by weight of the isomeric mixture, and isomer (II) constitutes the balance thereof, with (b) a hydroxyl-terminated dimethylsiloxane fluid of suitable molecular weight, e.g., 20,000-40,000 weight average molecular weight, to yield an acrylic-functional capped silicone polymer. The acrylic-functional capped silicone polymer then is compounded with filler, stabilizer, and a suitable photoinitiator such as benzophenone, to yield a photocurable silicone composition which is very rapidly cured in exposure to UV or other suitable actinic radiation.
The acrylic-functional capper employed to prepare the photocurable silicone polymer described above is typically prepared by selectively hydrosilylating propargyl acrylate at the propargyl group with dimethylchlorosilane. Subsequently, in the endcapping of the hydroxyl-terminated dimethylsiloxane fluid using the resulting isomeric mixture described above, hydrogen chloride is generated, necessitating the addition of a basic hydrogen chloride acceptor, e.g., an amine, to the reaction volume. The amine or other acid acceptor functions to prevent the acid from catalyzing the reversion of the dimethylsiloxane fluid.
While the above photocurable silicone system functions satisfactorily in many applications and has enjoyed wide commercial usage, it nonetheless suffers from several serious problems.
First, and of utmost importance, propargyl acrylate and its precursor, propargyl alcohol, are highly toxic, and thus require precautionary measures in handling and exposure, to ensure safety in their use.
Second, the process employed to make the acrylic-functional capper is complex and expensive. Propargyl alcohol is first esterified to make the propargyl acrylate, and the acrylate then is carefully hydrosilylated to form the acrylic-functional capper.
Third, the capping process, with a base such as an amine being employed as the hydrogen chloride acceptor, generates a base-complexed hydrochloride which is a solid. The formation of this solid complex necessitates the difficult step of filtering the solid from the viscous polymer, which adds to the cost of the process and the photocurable silicone product.
Fourth, the acrylic-functional capper comprises a constituent, viz., the isomer of formula (I) above, which as a .beta.-oxygenated silicon compound, is readily susceptible to undergoing .beta.-elimination reaction. For a discussion of .beta.-elimination chemistry, see C. Eaborn, "Organosilicone Compounds", Butterworths Scientific Publications, London, 1960, pp. 137-138. Thus, the isomer (I) of the capper mixture may undergo .beta.-elimination reaction to split off allene, thereby rendering the acrylic-functional capper unstable. Further, the resulting photocurable silicone composition comprising the photocured acrylic-functional capped silicone polymer may likewise be unstable due to the presence of the beta-oxygenated silicon linkage therein.
Apart from the above-described deficiencies of conventional photocurable silicone systems, these systems when fully cured are characterized by good toughness and hardness characteristics which in many applications are a decided advantage, however in a number of sealant, potting, and conformal coating applications, it is more desirable to utilize a soft, flexible, pliable and resilient cured material. The latter characteristics may be desired, for example, in applications involving substantial differential thermal expansion characteristics, such as where the silicone material is employed as a sealant between structural elements possessing substantially different thermal conductivity and heat capacity characteristics, in end use environments of rapidly changing temperature. Another end use application where such soft, flexible, pliable, and resilient character is desired, is the use of coatings on substrates or in elements which are expandable or otherwise must accommodate deformational stresses or pressures (e.g., gasket and bushing materials in high pressure hydraulic systems). In such applications, fully capped photocurable silicone compositions are excessively hard and inelastic, and their use entails the risk or occurrence of failure of the silicone materials, with consequent adverse effect on the efficiency or utility of the structure in which such materials are employed. In such applications, a silicone possessing soft, gel-like texture and accompanying flexibility would better serve the functional performance and physical property requirements for usage of silicone materials.
Accordingly, it would be a significant advance in the art to provide a photocurable silicone gel composition which can be made without highly toxic reagents in a relatively simple and economic manner, does not require filtering of a viscous polymer product, and comprises a silicone polymer which is capped with an acrylic-functional capper having no .beta.-oxygenated silicon linkages in its structure.
With respect to the photocurable silicone gel composition, precursor thereof, and process of making same which constitute aspects of the present invention and are more fully described hereinafter, related art to the present invention is discussed below.
"Synthesis of Organosilicon Esters of Hydrozino Carboxylic Acids," G. S. Gol'din, et al, Zhurnal Obshchei Khimii, Vol. 43, No. 4, April, 1973, pp. 781-784, discloses the synthesis of 1,3-bis[(acryloyloxy)methyl]- and 1,3-bis[(methacryloyloxy)-methyl]-1,1,3,3-tetramethyldisiloxanes by heating mixtures of chloro(chloromethyl)dimethylsilane with acrylic and methacrylic acids in the presence of triethylamine as hydrogen chloride acceptor, with subsequent hydrolysis of the silyl ester: ##STR4## wherein R=H or CH.sub.3.
U.S. Pat. No. 4,845,259 to M. Arai, et al, assigned to Shin-Etsu Chemical Company, Ltd., and Shin-Etsu's Japanese Kokai Tokkyo Koho JP 01 14226, JP 63 179881, JP 63 185989, and European Patent Application EP 276986, describe the synthesis of a silyl acrylic acid diester of formula (III) above, by reacting potassium or sodium salts of acrylic acid with chloromethyldimethylchlorosilane. Such synthesis, however, has the disadvantage that the acrylic acid salts employed therein are relatively expensive. In accordance with the teachings of these patents, the silyl diacrylate compound (III) is hydrolyzed to form a silanol capper of the formula: ##STR5## Contemporaneously, dimethylchlorosilane is reacted, in the presence of a platinum hydrosilation catalyst, with a vinyl-terminated dimethylsiloxane polymer to yield a product silicone polymer with chloro terminal groups. The silicone polymer is reacted with the silanol capper in the presence of amine to yield an acryloxymethyl-terminated polymer of the formula: ##STR6## and thereafter the product is filtered to remove the concurrently formed amine hydrochloride complex therefrom.
U.S. Pat. No. 4,563,539 to G. A. Gornowich, et al describes UV-curable acrylofunctional silicones which are formed by reaction of aminoalkyl or diaminoalkyl silicones with isocyanato acrylates.
Other relevant acrylic functional silicone references include: U.S. Pat. No. 4,503,208 (preparation of acrylate and 2-alkyl acrylate silicones which are curable by UV exposure, heat, or anaerobic conditions, by hydrosilation of an acrylate or 2-alkyl acrylate ester of an acetylenic alcohol with silicon hydride functional silicones); U.S. Pat. No. 4,575,546 (radiation-curable silicone polymers with a plurality of acrylic groups clustered at or near the chain ends thereof); U.S. Pat. No. 4,575,545 (same); U.S. Pat. No. 4,675,346 (silicone resin with terminal acrylic groups and intermediate region free of acrylic groups, formulated with fumed silica filler and photoinitiator, and curable by UV radiation); U.S. Pat. No. 4,504,529 (graft polymers having o-alkyl acrylate functionality, formed as a reaction product of a silicon hydride grafting agent with at least one o-alkyl acrylate group, and an aliphatically unsaturated polymer, e.g., a polyorganosiloxane); and U.S. Pat. No. 4,655,147 (methacrylated siloxanes prepared by hydrosilation of beta(allyloxy)ethylmethacrylate using a silicon hydride functional siloxane).
Photocurable siloxane rubber compositions are described in Japanese Kokai Tokkyo Koho JP 01 301708, and European Patent Application 0 240 162 A2 describes liquid polyorganosiloxane compositions that cure by a hydrosilation reaction to yield optically transparent elastomers.
Accordingly, it is an object of the present invention to provide a photocurable silicone gel composition and precursor thereof which may be synthesized using relatively low cost, readily available materials, which are relatively simple in synthesis procedure, and which avoid the necessity of filtering a highly viscous silicone polymer product to remove by-products therefrom.
It is another object of the invention to provide a photocurable silicone composition in which the silicone polymer is free of .beta.-oxygenated silicon linkages, and whose synthesis can be carried out without highly toxic reactant materials.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.